Tone interrupter for audiometers



Jan. 13, 1953 F. E. BARRON TONE INTERRUPTER FOR AUDIOMETERS Filed Oct.4, 1950 IIIIIIIIIIIIII E NN nr F m mw wwf.

Patented Jan. 13, 1953 TONE INTERRUPTER FOR AUDIOMETERS Fred E. Barron,Minneapolis, Minn., assignor to The Maico Company, Incorporated,Minneapolis, Minn., a corporation of Minnesota Application October 4,1950, Serial No.188,324

6 Claims.

My present invention relates to improvements in audiometers which aredevices, instruments,

apparatus or systems used in conjunction with suitable audio frequencyreproducers of the air conduction type or bone conduction type to testthe hearing characteristics of the human ear. Such audiometerscustomarily employtwo sources of signal energy (one of which is in thenature of a suitable variable frequency signal generator and the otherof which is in the nature of a microphone), a suitable audio frequencyampliiier intermediate the said signal energy sources and the electricaloutput of the audiometer, and means for selectively connecting the twosignal sources independently to the imput of the amplifler.

In an audiometer, it is necessary to provide some means for periodicallyinterrupting the test tone signal produced by the signal generator, andit is also highly desirable to provide means for similarly interruptingthe speech signal delivered by the microphone when the latter is in use.Heretofore, the usual procedure for interrupting tone producing signalenergy from the variable frequency signal generator has been to renderthe generator inoperative. Since signal generators used in audiometersare generally of the vacuum tube oscillator variety, the usual procedurehas been to manipulate one way or another to stop oscillation of theoscillator tube, and this has been most frequently accomplished byinterrupting the plate circuit of the oscillator tube, or by introducinginto the grid circuit of a signal generating oscillator tube a D. C.voltage for the purpose of reducing the mutual conductance of the tubeto a point where it will not oscillate.

These prior art tone interrupters of the kind which operate directly onthe signal generator or oscillator tube thereof have several inherentdisadvantages, two of which are as follows, to wit:

(A) They will not interrupt the microphone speech circuit, as isdesirable in some types of malingering tests, due to the fact that themicrophone and signal generator each comprise a complete and separatesource of signal energy; and

(B) In audiometers of the so-called Z-channel type, which employ twoseparate and distinct signal channels, one for use with air conductionreproducers and the other for use in bone conduction reproducers, andwherein both said channels are driven from a common tone-producingsignal generator or oscillator, it is obviously impossible toselectively interrupt the signal energy in one of the channels withoutalso interrupting the signal energy in the other of the channels.Because of this fact, it has been necessary, in order to achieveselective independent interruption of signal energy in the airconduction and bone conduction channels of 2`channel audiometers, bydirect operation on the signal, generator, to employ two signalgenerators, one for the air conduction channel and the other for thebone conduction channel. However, this practice has been foundobjectional for several reasons, including the inherent diillculty ofobtaining and maintaining perfect frequency balance between the twosignal generators, particularly when these are of the vacuum tubeoscillator type.

One of the primary objects of my invention is to overcome objection A,above, by the provision of an improved tone signal interrupting means orsystem, whereby the signal energy to the electrical output of anaudiometer, from any one of a plurality of signal energy sources, suchas the speech microphone and a signal tone producing generator, may beselectively and independently interrupted.

Another'primary object of my invention is to overcome objection B,above, by provision of an improved tone signal interrupting means orsystem, whereby the signal energy in either of the two channels of the2-channel audiometer, of the kind employing a single signal generatorfeeding the two channels, may be selectively and independentlyinterrupted.

Another important object of the invention is the provision of animproved signal interrupter, of the general character described, whereinthe fade-in and fade-out time is variable by variations in values ofcomponent elements of the system.

A further object of the invention is the pro- .vision of an improvedsignal energy interrupting system or means for audiometers having, inaddition to the advantages above noted, the inherent ability tocompletely eliminate signal energy at ,the output when the toneinterrupting system or means is operative.

A still further object of the invention is a provision of an improvedsignal energy interrupting system or means of the class described,whereby the signal is faded in and faded out without generation ofextraneous clicking or other noise.

The above and other important objects and advantages of the inventionwill be made apparent from the following specification, claims andappended drawings.

Referring to the drawings, the single view is a schematic -electricaldiagram of a 2-channe1 audiorneter incorporating a preferred embodimentof my invention.

In the drawing, two separate and distinct sources of signal energy areillustrated, one being in the nature of a variable frequency signalgenerator I, and the other being in the nature of a speech microphone 2.The variable frequency signal generator I may be of any suitable type,but will usually be, and for the purpose hereof may be assumed to be, ofa type employing a` vacuum tube oscillator.

One side of the output of the signal generator I is connected to groundb-y a lead 3 and the other side of the output of signal generator I isconnected by a lead 4 to a contact 5 of a selector switch 5. Similarly,one side of the output of the microphone 2 is connected to ground by alead 'I and the other side is connected by a lead 8 to a` contact 9 ofselector switch 6. The selector arm or movable -contact of switch 6 isindicated by IB and is connected to the air conduction signal energychannel by a lead II and to the bone conduction signal energy channel bya lead I2. The air conduction signal energy channel, as shown, comprisesa suitable signal energy balancing controller I3 for modifying thecharacteristic power output curve of the signal generator to meet testrequirements in connection with an air conduction reproducer, a poweramplifier I4, and a power output transformer I5. The bone conductionsignal energy channel, as shown, comprises a suitable signal energybalancing controller I6 for modifying the characteristi-c power outputcurve of the signal generator to test requirements in connection with abone conduction reproducer, a power amplifier I'I, and an outputtransformer I8. The signal energy balancing controllers I3 and I6 may bein the nature of the usual variable attenuators conventionally used forthe purpose. Ihe power amplifiers I4 and I'I, as shown, and preferably,are each of the 2-stage variety afforded by a twin triode vacuum tubeamplifier. The input section of the twin triode amplifier tube I4comprises element i9, a cathode element 20, a cathode heater 2I, and acontrol grid 22. The output section of amplifier I4 comprises a plateelement 23, a cathode element 24, a cathode heater 25, and a controlgrid element 2B. The input section of the twin triode amplifier I'Icomprises a plate element 2l', a cathode element 28, a control gridelement 29, and a cathode heater 30. IIhe output section of the amplierI'I -comprises a plate element 3|, a cathode element 32, a control gridelement 33, and a cathode heater 34.

The signal output energy from the signal energy balancing controller I3of the air conduction signal channel is connected to the control grid 22of the input section of amplifier I4 by a lead 35 having interposedtherein a condenser 3E.. The output of the signal energy balancingcontroller IB of the bone conduction signal channel is connected to thecontrol grid 29 of the input section of amplifier I'I by a lead 31having interposed therein a condenser 38. The cathodes 23 and 24 ofamplifier I4 and the cathodes 28 and 32 of amplifier Il are each-connected to ground through a self-biasing resistor 39 and by-passcondenser l. The plate I9 of the input section of amplier Id isconnected to the positive side of a suitable source of high voltage D.C., such as a battery B, through a lead III having interposed therein aload resistor 42. The plate 21 of the input section of amplifier I'I issimilarly connected to the positive side of battery B by a lead 43having interposed therein a load resistor 44 and part 4 of lead 4I.'I'he plate 23 of the output section of amplifier I4 is connected to thepositive side of battery B by a lead 45 and the primary winding i6 ofcoupling transformer I5. The plate 3I of the output section of amplifierI1 is connected to the positive side of battery B by a lead lil, theprimary winding 48 of -coupling transformer i8, a lead 33, part of leadI3 and part of lead 4I. One end of the secondary winding 55 of outputtransformer I5 is grounded at 5I, and the corresponding end of secondarywinding 52 of transformer I8 is grounded at 52. The secondary winding5I] of transformer I5 of the air conduction signal channel is adapted tobe connected and may be assumed to be connected to a suitable outputpower attenuator and air conduction reproducer. Similarly, the oppositeends of the secondary winding 52 of coupling transformer IS are adaptedto be and may be assumed to be connected to a suitable output powerattenuator and bone conduction reproducer. The plate I9 of the inputsection of amplifier I4 is connected to the control grid 26 of theoutput section of the amplifier III through a lead 53, having interposedtherein a coupling condenser 5. Similarly, the plate 2l of the inputsection of amplifier Il' is coupled to the control grid 33 of the outputsection of amplifier I'I by a lead 54'.' and coupling condenser 55. Theamplifiers Iii and Il may properly be referred to as cascade amplifiers,since the input and output sections thereof are cascade coupled.

As a means for selectively interrupting the test signal energy in theair conduction signal channel of the audiometer, I provide an improvedcontrol system or means which, in its preferred embodiment illustrated,comprises as major cornponents a normally open interrupter switch 56, anelectronic amplifier in the nature of a triode vacuum tube 51, a sourceof high voltage D. C. B2, rectifiers 58 and 59, and a suitable source ofA. C., such as a transformer 35. The amplier 51 comprises a plateelement Si, a cathode element 52, a cathode heater 63, and a controlgrid element Eli. The heater G3 of amplifier 5l is energized from thesecondary winding 55 of transformer 60 to which it is connected by leads65 and 61. The primary Winding 58 of transformer 69 is energized from asuitable high Voltage source, such as an A. C. power line G9. The platecircuit for amplifier 5i comprises battery B2, a load resistor It fromthe positive side of battery B2 to plate 6I through lead il, cathode 52and a self-biasing resistor 'I2 connected between cathode E2 and thenegative side of battery B2 through a grounded lead 73. The platecircuit of amplifier tube 5I is connected to the output section controlgrid 26 of amplifier it! by a lead 'I4 having interposed therein acoupling condenser 15, the rectifier 58, filter resistor I5, a lead TI,a filter resistor '18, a lead T9, and a grid bias resistor 80. Thepositive side of the rectifier 58 faces the plate 6I and the negativeside thereof faces the control grid 26 of amplifier llt. In thepreferred arrangement illustrated, the coupling Acircuit between theplate 6I of amplifier 57 and the grid 25 of amplier I4 is of the voltagedoubling variety and further comprises the rectifier 58 having itsnegative side connected to the positive side of rectifier 58 by a lead 8I, and its positive side connected to grounded lead i3. In thearrangement illustrated, a lter resistor 52 and filter condenser 83 areconnected between lead 'Il and grounded lead "I3, and a filter condenser8d is connected between lead I9 and ground lead 'I3 by a lead 85. Thecontrol grid 64 of amplifier 5l is connected to ground by a grid biasresistor 86. For the purpose hereof, the control grid 64 of amplifier 51may be excited from any suitable source of signal energy, including thesecondary winding 65 of the cathode heater energizing transformer 60. Asillustrated herein, the transformer secondary winding 65 is connectedacross the grid bias resistor 86 by ground connections from one end ofwinding 65 to the grounded end of resistor 8B and by a lead 81 to theother end of resistor 86 and grid 64, and which lead has interposedtherein the normally open interrupter switch 56.

As a means for selectively interrupting the signal energy in the boneconduction signal channel of the audiometer, I provide an improvedcontrol system or means substantially identical to that described inconnection with the air conduction signal channel and which, in thepreferred embodiment hereof herein illustrated, -comprises as majorcomponents a normally open interrupter switch 88, a triode electronicamplifier tube 89, a high voltage source of D. C. in the nature of abattery BI, a relatively low voltage source of A. C., in the nature of atransformer 90, and rectifiers 9| and 92. The primary winding 93 of thetransformer 90 may be energized from any suitable source of A. C., suchas the high voltage A. C. supply line 69. The amplifier 89 comprises aplate element 9d, a cathode element 95, cathode heater 96 and a controlgrid element 91. The cathode heater or filament 96 is -connected acrossthe transformer secondary winding 98 by leads 99 and |90. The controlgrid 91 may be excited from any suitable source of signal energy, whichmay expediently be, and is shown as being, the secondary winding 98 oftransformer 90. For connecting the secondary winding 98 across the gridbias resistor IOI, I provide a lead |02 from one side of secondarywinding 98 to grid 91 and the ungrounded end of resistor IGI, and havinginterposed therein the normally open interrupter switch 88. The otherend of secondary winding 93 is connected to ground through a groundedlead |03. The negative side of battery BI is connected to ground throughlead |03. The plate circuit for amplier 89 comprises a load resistor |95from the positive end of battery BI to plate 94 through a lead |95,cathode 95, and a selfbiasing resistor |66 from cathode to ground andBl. A plate circuit of amplifier 89 is connected to the control grid 33of the output section of audiometer amplifier I1 by the lead |05,coupling condenser IGB, a lead |61, rectifier 9|, filter resistor |99, alead I I9, filter resistor I|I a lead ||2, and a grid bias resistor II3.The adjacent ends of resistors I 09 and III are connected to ground lead|93 by a filter resistor II4 and filter condenser II5, and the other endof resistor III is connected to ground through a filter condenser I I3.In the preferred arrangement illustrated, the positive side of rectifier9| is connected to ground through the voltage doubling rectifier 92.

Operation When it is desired to utilize the signal generator I as aSource of test signal energy, the switch arm I6 of selector switch 6 ismoved into engagement with contact 5 of switch 6, and when it is desiredto utilize the speech microphone 2 as a source of test signal energy,the selector arm I0 of selector switch 6 is moved into engagement withcontact 9 of said switch.

In either event, the signal energy from the connected source I or 2 willbe conducted to the air conduction signal energy channel through leadergy thus conducted to the air conduction signal energy channel throughlead II will be acted upon by the balancing controller or attenuator I3,which will vary the output power characteristic of the source atdifferent frequencies in a manner desirable for use with an airconduction reproducer. The signal energy similarly conducted to the boneconduction signal energy channel through lead I2 will be similarly actedupon by the balancing controller or attenuator I6 in a manner which willmodify the signal energy power at different frequencies, as is desirablefor use in connection with a bone conduction reproducer. The airconduction signal energy channel will then be impressed upon controlgrid 22 of the input section of amplifier I4 through lead 35 andinterposed coupling condenser 36; and the bone conduction signal energychannel will then be impressed upon control grid 29 of the input sectionof amplifier I1 through lead 31 and interposed coupling condenser 38.

Provided that the interrupter switches 56 and 88 are both open, asshown, the amplifiers 51 and 89 will be inoperative. Under theseconditions, the input and output sections of signal channel amplifiersI4 and I1 will be grid biased to operate normally or on the linearportions of their characteristic plate current curves. The signalvoltage thus impressed on control grid 22 of the input section ofamplifier I4 will be reflected in and amplified in the plate circuit ofthe input section of amplier I 4, which latter comprises source ofpotential B, lead 4I, load resistor 42, cathode 20 and grid bias,resistorI 39. Similarly, the signal voltage thus impressed on controlgrid 29 of the input section of amplifier I1 will be refiected in andamplified in the plate circuit of the input section of amplifier I1, andwhich latter comprises the source of potential B, part of lead 4|, lead43, load resistor 44, cathode 28 and grid bias resistor 39. Theamplified signal voltage in the plate circuit of the input section ofamplifier I4 is impressed upon the control grid 26 of the output sectionof amplifier I4 through lead 53 and interposed coupling condenser 54;and the amplified signal voltage in the plate circuit of the inputsection of amplifier I1 is similarly impressed upon the control `grid 33of the output section of amplifier I1 through the lead 54 and interposedcoupling condenser 55. The amplified signal voltage thus impressed uponthe control grid 26 of the output section of amplier I4 is reflected inand amplified in the plate circuit of the output section of amplifier I4, which latter comprises the primary winding 46 of coupling transformerI5; and the amplified Isignal voltage thus impressed on the outputsection control grid 33 of amplifier I1 is reflected in and amplified inthe plate circuit of the output section of arn-z plifier I1, whichcomprises a primary winding 48 of coupling transformer I 8. Hence, theoutput signal energy fol` use with an air conduction reproducer will beinduced in the secondary winding 50 of transformer I 5 and output signalenergy for use with a bone conduction reproducer will be induced in thesecondary winding 52 of coupling transformer I8.

If it now be desired to selectively and independently interrupt theoutput signal energy in the air conduction signal channel comprisingamplifier I4, the normally open interrupter switclf56 will be closed.When this is done, the A. C. voltage in the secondary winding 65 ofaces-,cie

7. transfer-merlin will be applied: across grid bias resistor 86 andimpressed upon control grid 613` of amplifier 51, to excite the latter;The A. C.A voltage thus impressed upon the control grid E4 of amplier 51will be reflected in and amplified in the plate circuit of amplier 51comprising load resistor 1i! and lead 1I. The amplified signal voltagein the plate circuit of amplier 51 will -be rectified and increased invalue in the voltage doubler circuit comprising coupling condenser 15,rectiers 58 and 59, and lte'rcondensers 83 and 8, and will be applied tothecontrol grids 22 and 2G of amplifier I4 as a'negative D. C. grid biaspotential of sufficient value to bias the plate circuits of the inputand output sections of amplifier I4 to cut on. In this respect, it willbe noted that negative D. C. biasing potential is applied, as indicated,to grid 26 of amplifier I6 through the lead 19 and the grid biasresistor Sli, and that the said negative D. C. control potential isapplied to control grid 22 of amplifier I4 through part of said lead 19,a lead 19 and a grid bias resistor 86.

To selectively and independently interrupt the signal energy in the boneconduction signal energy channel, the control grid 91 of ampliner 89 isexcited with A. C. by closing interrupter switch 88. The A. C. voltagethus impressed upon control grid 91 of amplier 8d is reflected in andamplied in the plate circuit of amplifier 89 comprising load resistor HMand lead I. The amplied signal voltage of the plate circuit of amplier39 is rectified and further amplied in the voltage doubler circuitcomprising condenser IOS and rectiiiers 9| and 92, is filtered by thefilter network comprising filter resistors H39, Ill, H4 and iiltercondensers H5 and IIS, and is impressed upon the control grids 29 and 33of amplifier I1 as a negative D. C. .grid biasing and' control.potential of suflicient value to bias the input and output sections ofamplifier I1 to plate current cut off. The rectied and filtered negativeD. C. control potential is applied to grid 33 From the above it willneevident that thel invention provides highly eiicient means or systemfor selectively and independently interrupting the signal energy ineither of thetwo channels of a 2-channel audiometer of the-kind whereinboth channels are operated from al common source of signal energy. Itwill be further evident that my improved signal interrupting means orsystem serves equally well in connection with either of two sources ofsignal energy, such as the signal generator I and the speech microphone.

important advantages arising from the use of 2-stage amplifiers, such asshown at I4 and Il, and the interruption of the signal through theseamplifiers by simultaneous application of cut oif control potential tothe input and output sections of these amplifiers, are as follows, towit:

(A) The magnitudev of the signal from the source, if not altogethereliminated in tl'iein-putY 82 the eiected signal channel are completelycut off.

With' the signal interrupting system or means of myinvention, the signalchannels are interrupted smoothly and silently without the production ofany extraneous noise and the fade-in and fade-out time intervals may becontrolled by varying values of component circuit elements, such forexample as the condensers 15, 83, 84, Hi8', H5', IIB, and resistors 16,18 82, |09, III and. I I4.

My invention has beenthoroughly tested and found to be'entirely adequatefor the accomplishment of 4the objectives set forth; and, while IV haveshown a preferred embodiment of my invention, it Will'be understood thatthe same is capable of modification without departure from the spiritand scope of the invention as dened in the claims.

What I claim is:

l. In an audiometer, a normally operative first electronic signalamplier having plate and cathode and control grid elements, a platecircuit for the rst amplifier coupled to the electrical output'of theaudiometer, a source of test signal energy coupled to the control gridof the rst amplier to drive the same, and normally inoperative controlmeans for biasing the first amplifier to plate current cut off tothereby interrupt the delivery of signal energy to the electrical outputof the audiometer, said control means comprising a second electronicamplifier having plate and cathode and control grid elements, a normallyopen control grid driving circuit for the second ampliiier, a manuallyoperative interrupter switch controlling the grid driving circuit of thesecond amplier, a plate circuit for the second amplifier, and a D. C.control circuit for the rst amplifier comprising a rectifier coupledbetween the plate circuit of the second amplifier and an element of therst amplier other than the plate thereof for applying a D. C. controlpotential to the iirst amplifier responsive to closing of theinterruptor switch and resultant energization of the control griddriving circuit of the second amplifier, the polarity and value of theD. C. control potential thus applied to thesaid element of the rstamplifier being such as to bias said r'st amplier to cut olf.

2; In an audiometer, a normally operative rst electronic signal energyamplier having plate and cathode and control grid elements, a platecircuit for said first amplifier coupled to the electrical output of theaudiometer, a source of test signal energy'coupled to the control gridelement of the first amplifier to drive the same, and normallyinoperative control means for biasing the rst amplifier to plate currentcut off to thereby interruptdelivery of signal energy to the electricaloutput of the audiometer, said control means comp-rising a secondelectronic amplier having plate and cathode and control grid elements, anormally open control grid driving circuit for the second ampliiier, amanually operative interrupter switch controlling the said grid drivingcircuit of the second ampliiier, a plate circuit for the secondamplifier, and a rectier having its positive side coupled to the plateof the second amplier and its negative side coupled to the control gridelement of the first amplifier, whereby when the grid driving circuit ofthe second amplifier is closed a negative D. C. control potential willloe-applied to the control gridof the rst amplier as a grid bias, thevalue of the negativeD. C.' control potential thus ap- 9 plied to thecontrol grid of the first amplifier being sufficient to bias saidamplifier to plate current cut off.

3. In an audiometer, a normally operative first electronic signalamplifier having plate and cathode and control grid elements, a platecircuit for the first amplifier coupled to the electrical output of theaudiometer, a source of test signal energy coupled to the control gridof the first amplifier to drive the same, and normally inoperativecontrol means for biasing the rst ampliiier to plate current cut off tothereby interrupt the delivery of signal energy to the electrical outputof the audiometer, said control means comprising a second electronicamplifier having plate and cathode and control grid elements, a normallyopen driving circuit for the control grid element of the secondamplifier comprising a source of A. C. potential and a normally opensignal energy interruption control switch, a plate circuit for thesecond amplifier, a first rectifier having its positive side connectedto the plate of the second amplifier through the medium of a couplingcondenser and its negative side connected to the control grid element ofthe first amplifier, and a second rectifier having its negative sideconnected to the positive side of the first rectifier and its positiveside connected to the negative side of the plate circuit of the secondamplifier, whereby when said signal interruption control switch isclosed, amplified signal energy in the plate circuit of the secondamplifier will be increased in value, rectified and applied as anegative D. C. grid bias potential to the control grid of the firstamplifier, the value of the potential thus applied being sumcient tobias the first amplifier to plate current cut off.

4 In an audiometer, a normally operative first electronic signalamplifier having plate and cathode and control grid elements, a platecircuit for the first amplifier coupled to the electrical output of theaudiometer, two sources of test signal energy, switch means forselectively and singularly coupling the two sources of signal energy tothe control grid element of the first amplifier to drive the same, and anormally inoperative control system for biasing the rst amplifier toplate current cut off to thereby interrupt delivery of signal energy tothe electrical output of the audiometer, said control means comprising asecond electronic amplifier having plate and cathode and control gridelements, a normally open control grid driving circuit for the secondamplifier having interposed therein a normally open interrupter switch,a plate circuit for the second amplifier, a first rectier having itsnegative side coupled to the control grid of the first amplifier, acondenser interposed between the positive side of said rectifier and theplate element of the second amplifier, and a second rectifier having itsnegative side coupled to the positive side of the rst rectier and itspositive side connected to the negative side of the plate circuit of thesecond amplifier.

5. In an audiometer, a normally operative twostage amplifier, each stageof which has plate and cathode and control grid elements, a source oftest signal energy having its output coupled to the control grid of thefirst stage of said twostage amplifier to drive the same, a platecircuit for the first stage of said two-stage amplifier, means couplingthe plate circuit of the first stage `of said amplier to the controlgrid element of the second stage of said amplifier to drive the same, aplate circuit for the second stage of the 2-stage amplifier coupled tothe electrical output of the audiometer, means normally biasing thefirst and second stages of said amplifier to operate at normal platecurrent, and a normally inoperative signal energy interrupter for saidaudiometer comprising a second electronic amplifier, a normally opencontrol grid driving circuit for the second amplifier having interposedtherein a normally open interrupter switch, a plate circuit for thesecond amplifier and a rectifier having its positive side coupled to theplate of the second amplifier and its negative side coupled to thecontrol grids of the first and second stages of the first-said amplifierto apply a negative D. C. grid biasing potential to the said grids ofthe first amplifier, the value of the D. C. biasing potential thusapplied to the control grid elements of the rst and second stages of thefirst amplifier being sufficient to completely eliminate the signalvoltage in the plate circuit of the second stage of said amplifier.

6. In a multiple channel audiometer, a first signal channel comprisingan electronic amplifier, a second signal channel comprising anelectronic amplifier, a signal generator common to the two saidchannels, means for coupling the output of the signal generator to thecontrol grid of each of said amplifiers to drive the same, a platecircuit for each said amplifier coupled to the electrical output of thesignal channel of which said ampliiier is a part, and a normallyinoperative manually controlled signal interrupter system for each saidsignal channel, each said interrupter system comprising an electronicamplifier having plate and cathode and control grid elements, manuallycontrolled circuit means for driving the control grid of each controlsystem amplifier, each such means comprising a manually-operatedinterrupter switch, a plate circuit for each said control systemamplifier and circuit means coupling the plate circuit of each saidcontrol system amplifier to an element of a different one of said signalchannel amplifiers other than the plate element thereof, each last saidcircuit means comprising a rectifier for rectifying the currentimpressed upon the coupled element of the signal channel amplifier, theD. C. control potential thus applied to the coupled elements of lthesignal channel amplifiers being of proper polarity and sufficient valueto reduce the amplification factor of that amplifier to a point wherethe signal energy will be eradicated in the output of :that signalchannel.

FRED E. BARRON.

REFERENCES CITED The following references are of record in the file ofthis patent:

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